The present invention relates to an apparatus and method for measuring power factor and torque on the output of variable frequency drives.
Adjustable speed drives have been in use for several years and they are widely used for controlling the speed of induction motors. Such adjustable speed drives offer several advantages over fixed speed drives For example, adjustable speed drives increase the productivity of industrial machines since the machine speed can be selected for maximum output consistent with good product quality. Adjustable speed drives also make industrial machines more flexible so that when a product change requires a different drive speed, the new speed is easily selected. This benefit eliminates the need for gear or belt ratio changes. Furthermore, electronic adjustable speed drive systems together with process or programmable controllers enable the controlling of machine speed, fan speed or pump speed thereby further increasing the productivity and versatility of any machine utilizing such adjustable frequency drives.
In general, motor speed can be controlled by varying factors such as line frequency, the number of motor poles, and motor slip. By controlling the motor speed by varying the frequency, a continuously variable, highly efficient control throughout the entire speed range may be achieved. Furthermore, such a control system is applicable to widely used, three-phase squirrel-cage motors.
Variable frequency may be provided to input terminals of an AC motor in the following manner. A main three-phase supply is first rectified and smoothed in a rectifier or converter section. This DC power is then fed into an inverter section, the current output of which approximates a sine wave of variable frequency and amplitude.
Two different methods of obtaining the variable frequency output may be used. In a six step system, Pulse Amplitude Modulation (PAM), the DC voltage obtained in the converter is varied. In a Pulse Width Modulation system (PWM), the frequency and the voltage are controlled by varying the pulse width within the invertor. The current output waveshape of the PWM system is superior to output wave forms produced by the PAM system. For both types of systems, the goal is to generate a current waveform that approaches sinusoidal with the harmonic components of the waveform at a minimum to ensure minimized torque pulsation and temperature rise. In neither case, however, has this goal been completely met by known devices.
In many situations, the effectiveness of a drive can be further increased by accurately measuring the power output. Measuring the instantaneous power input to a machine or process provides a great deal of valuable information. This measurement which can be used as a feedback signal may be utilized to: automatically adjust the machine feed rate; signal the beginning or end of a process; detect malfunctions or problems; and, indicate, without contact, the flow rate, viscosity or pressure.
Load controls that sense power, have set points and analog outputs are widely used in machine tools, chemical processes and material handling. Unfortunately, such load controls do not work on variable frequency sources. In order for a variable frequency power sensor to be of practical use as a machine controller, the sensor must have the ability to: accurately measure power at both low and high frequencies; provide a fast response with a low ripple and immunity to noise; and, have the capability of working on both pulse amplitude modulation drives and pulse width modulation drives.
The main use of variable speed drives is to power induction motors. To measure this power, the lag of the current behind the voltage (or power factor) must be considered. Traditional watt sensors rely on sensing the zero crossing of the sinusoidal voltage and current for power factor calculation. Typical waveforms from PWM and PAM drives show that the waveform is not clean enough for precise zero crossing measurement. A secondary problem is the measurement of current. Many sensors use a current sensing toroid or a lamination transformer, but these devices are not reliable at low frequencies. The combination of zero crossing and current measuring difficulties means that typical watt transducers do not work on the output of a variable frequency drive.
The measurement of AC power requires the multiplication of voltage, current and a power factor so that the equation is: EQU P=V.times.I.times.Cos .phi.
A simple and reliable method for performing this computation electronically is by means of a Hall generator. A Hall generator is a magneto-sensitive semiconductor which, when driven by an electric current and exposed to a magnetic field, generates a voltage that is proportional to the product of current and field. To utilize a Hall generator to measure power, a Hall device excitation current I is derived from a line voltage, and the phase load current produces a proportional field B in the magnetic circuit. The Hall generator exposed to this field generates an output voltage proportional to the product of I, V, and the phase angle between them. The output contains an AC component and a DC component. The AC component can be filtered out if necessary, and the expression for the DC component is V=k.times.I.times.B=k.times.V .times.Icos .phi.=k.times.P where k is a constant representing the Hall voltage. The DC output voltage is therefore a measure of the AC power. With such additional power measurements as described above, only one or two phases are measured, and as a result, there is a large ripple component in the resulting output.
In three phase power measuring devices which are used for fixed frequency power sensors, either one or two transducers are utilized to measure either one phase under the assumption that the load is balanced or two phases, respectively. A computer simulation of either of these approaches at various power factors shows that the output would have a large ripple component which is unacceptable for control operations.
For many control applications, fast response is also critical. Typical response time for watt transducers is 250 to 500 milliseconds. The slow response is due in great part to filtering circuits. For a power sensor to be useful the response time should be reduced to about 15 milliseconds.
A power transducer must also live in close proximity to the variable frequency drive, and such drives generate a great deal of RF noise from the high frequency switching. Therefore, both the housing of the sensor and the internal circuitry should be designed to minimize RF noise.
In addition to sensing power of the variable frequency drive, it is often important to measure the torque produced by the drive. Torque is equivalent to the horsepower divided by the speed multiplied by a constant. After measuring the power produced by the variable frequency drive, a measurement of the speed of the drive through a speed transducer and dividing this into the horsepower allows one to determine the torque. Existing schemes for measuring torque are usually mechanical or implemented electronically with slow power transducers. Torque measurements are important in industry as they form the basis for understanding many mechanical phenomenon, and in particular are used to characterize electric motor performance.
Relative to electric motors, mechanical torque measurement methods fall into three general categories:
1. All mechanical. PA1 2. Electronic strain gauges mounted on mechanical members. PA1 3. Eddy current brakes.
In case 1, the classic Pony brake applies a friction load to the output shaft by means of woodblocks, flexible bands, or other friction surface devices. The torque is then measured by balancing the outputs against weights applied to a fixed lever arm. In case 2, a strain gauge is mounted directly to the shaft transmitting the power in the load. The shaft twists as a function of torque, the strain gauge deforms and a voltage output proportional to the torque results. In case 3 a rotating metal disc in a magnetic field induces eddy currents in the disc. These currents dissipate as heaL providing a value equal to horsepower; a tachometer provides a speed reference and division of the two results in torque measurement.
Methods 1 and 3 discussed above work fine for measuring torque but are not practical in all applications. The devices required to implement these methods are physically large, measure the torque output very slowly, and are cumbersome to implement. Strain gauges can be made quite small and do respond rapidly, however, they have reliability problems associated with the wiring to the resistive bridge since the bridge is mounted on a rotating surface. Schemes for brush pickups or RF or inductive coupling have been used, but result in increased cost, slower response, more complexity and greater physical size. Installation costs are also high since it usually requires modification of the machine.
Another useful measure of variable frequency drive performance is power factor. There are many methods to compute power factor, but they all have shortcomings Generally they only work at one frequency (e.g. 60Hz), look at only one phase of a three phase system and only work with sinusoidal waveforms. With variable speed drives, distorted wave shapes are common and do not fare well with conventional power factor measurement techniques. Furthermore, most systems use a great deal of damping so that rapid or instantaneous measurements of power factor are not possible.
It is therefore a principal object of the present invention to provide an apparatus and method for sensing torque and power factor of a variable frequency drive that is accurate, reliable and which provides an output signal that does not exhibit a large ripple component.
It is another object of the present invention to provide an apparatus and method for measuring torque and power factor of a variable frequency drive which will be sensitive at both low and high frequencies and will provide a fast response time.
A further object of the present invention is to provide an apparatus and method for measuring torque and power factor of a variable frequency drive which Will provide independent and precise machine control and protection.
A still further object of the present invention is to provide an apparatus and method for sensing torque and power factor of a variable frequency drive which will provide a linear output and which is extremely forgiving to gross overloads.
Yet another object of the present invention is to provide a system for determining torque on either fixed frequency sources or variable speed drives.
A still further object of the present invention is to provide a method to compute power factor which works at more than one frequency.
Still another object of the present invention is to provide a method to compute power factor that will work on a variety of waveforms.
Yet another object of the present invention is to provide a method to measure power factor that is essentially instantaneous.